The mechanisms involved in amniotic fluid (AF) volume regulation and composition are currently poorly understood. Prior studies in the ovine fetus have demonstrated a direct route of absorption from the amniotic cavity into fetal blood. This route of absorption is via vessels on the placenta and within the vascularized fetal membranes and is termed intramembranous absorption (IMA). We are currently investigating whether IMA occurs in the monkey (a species with less vascularization within the membranes and between the placenta disks), and if its role in AF regulation can provide a model for the human fetus. Five chronically catheterized rhesus monkey fetuses (Macaca mulatta) at 1251 0.5 (SE) daysU gestation (term 165110 days) with ligated esophagi and catheterized trachea were studied. Samples of fetal and maternal blood (0.5 ml), and amniotic and lung fluid (0.5 ml) were obtained at 0, 15, 30, 60, 120, 180, and 240 minutes after injection of 0.1 mCi (3 ml) of technetium-99m (Tc-99m) into the amniotic cavity. Results indicated that in spite of esophageal ligation there was a rapid absorption of the Tc-99m into the fetal circulation within 15 minutes of injection. The maternal Tc-99m activity was noted to increase in parallel to the fetal activity but remained marginally lower. The fetal lung fluid Tc-99m activity increased more slowly but equaled the fetal level by 4 hours. This suggests that the source of the Tc-99m in the lungs is extraction from the fetal circulation as opposed to inhalation from the amniotic cavity. Based on these studies, we conclude that the rapid absorption of Tc-99m from the amniotic cavity demonstrated the presence of IMA in this species. Further studies with this model will provide essential insight into the role of the intramembranous pathway in the amniotic fluid volume regulation and composition.